In the offshore storage of liquids characterized by a density less than that of water, such as petroleum products, crude oil and the like it is both desirable and economical in many respects to store the liquids beneath the water's surface. Such storage, has the concurrent drawback, however, that there is a strong possibility of leakage of the petroleum products. Such leakage could permit passage of the lighter components to the water's surface and thereby possibly create a polluting condition.
As a practical expedient such holding of petroleum products prior to their being transferred to a vessel or piped to a shore installation, the underwater tank or storage facility is normally ballasted with the surrounding water as the lighter products are withdrawn. Further, as the lighter products are introduced to the tank, the ballast water is displaced from the main storage facility and normally discharged into the surrounding water.
The system as a whole is found to work effectively in maintaining the storage facility always in a full condition and wholly ballasted. However, at least one primary fault in the system persists. Thus, water which is discharged from the storage zone by incoming hydrocarbon liquid, tends to carry residual liquid along with it as the water is urged from the tank.
This residual oil or other lighter than water product will gravitate toward the water's surface where it could create a visible sheen or other discoloration and possibly a water polluting condition.
A number of ways have been suggested toward making this type of facility both practical and environmentally acceptable, by treatment of the water which is discharged from the storage tank to cause separation from the oil. Many of the systems presently known utilize some form of separator or skimmer arrangement which operates during the transfer operation to treat discharge water. After treatment, the water can usually be returned safely to the surrounding area.
Such treatment normally requires that the body of water to be treated possess a degree of quiescence rather than being in a turbulent or disturbed state. In the latter state there would be a tendency to mix the two liquids rather than permit the lighter component to float to the surface of the heavier component.
Toward providing a suitable means for effecting a separation of oil from displaced water, the present invention provides an underwater tank of the type suggested. Discharged ballast water is received in a relatively expansive settling area adjacent the storage tank such that it does have an adequate chance to reach a quiet state prior to being skimmed, separated, or in other ways removed from the supporting body of water.
A further advantage of this type of storage facility is overcome in that the storage means is provided with an insulating or buffer compartment. The latter contains water, first contacted, and will absorb the initial impact in an instance when the storage facility is inadvertently contacted by a moving vessel or other body. Thus, should such damage occur to the tank there will be no discharge of oil into the surrounding water; rather the jacketing tank will cushion external contact and serve to preserve the integrity of the inner member.
The instant invention is thus addressed to an underwater storage facility for concurrently holding varying quantities of water and petroleum products. While many products can be so stored, for the present description the terminology oil will be referred to as the stored product, which term further covers a host of hydrocarbon liquids. The latter, in any event, being lighter than water, will tend to rise to the surface of the water under all circumstances.
Physically the disclosed facility thus includes a central holding tank or compartment defined by liquid tight wall and a roof enclosure. The latter two in effect concurrently contain the two liquids in varying amounts as required, with a varying interface therebetween. Pumping means communicated with the holding or storage compartment regulate the amount or flow of oil which is transferred to or from the facility. A collecting tank is positioned to receive residual oil rather than permitting the latter to be discharged to the surrounding water.
One object of the invention therefore is to provide a subsea storage facility for concurrently holding both water and oil, said facility being equipped to foster separation of the two liquids. Another object is to provide a weir section communicated with the tank storage area whereby to promote further separation of oil from the water. A still further object of the invention is to provide a collector means communicated with the main storage facility whereby the lighter component liquid will tend to float to the surface and be withdrawn from the collecting area. Still another object is to provide a jacket about the liquid holding facility which will function to protect the latter in the event it is damaged by contact with a floating vessel or other object.